Trusses for the roof and floor of a dwelling typically comprise a series of lumber planks arranged in a triangulated pattern. Truss design varies from house to house because of consumer demand for individualized home design, but within a single home generally several trusses of identical configuration will be employed.
A particularly time-consuming task of truss construction is the "set-up" process, which is the placement of locator stops on a truss table into positions in which they force truss planks into the proper position and orientation for subsequent attachment. Each set of planks is precut to the proper length and end angle, but must be arranged on a truss table in the correct triangulated truss configuration prior to being fixed into that configuration with truss plates.
Because set-up can be so time consuming, often a set-up system is used to hasten the set-up process. A set-up system typically includes a number of locator stops, or "jigs", which are quickly positioned on the truss table in a pattern that defines the proper placement of planks on the table. The positions of the jigs are usually predetermined for the truss manually or by a software program associated with the set-up system. The planks are then arranged on the table, with their positions and relative orientations being defined by the positions of the set-up jigs. Semi-permanent locator stops are then placed around the planks based on the location of the planks. Generally, the truss table will include a grid of holes which receive pins that secure the locator stops; alternatively (if the truss table has a wooden support surface), the semi-permanent locator stops can be attached to the table surface with nails. These locator stops are considerably larger than the set-up jigs and are capable of fine translatory and angular adjustment so that pressure can be applied virtually anywhere on the planks to assure consistency between trusses.
Once the locator stops are properly positioned, the planks are attached to one another by a pressure roller or hydraulic press which presses a connecting truss plate into adjoining planks to form the truss. The completed truss is removed from the table, and another set of planks is guided into position within the locator stops. The locator stops remain in place until all trusses of the selected configuration have been formed. They are then removed, and the set-up jigs are once again placed on the table to quickly define the configuration for the next truss configuration.
Set-up systems with positioning capability have been constructed in a variety of configurations. For example, U.S. Pat. No. 5,085,414 to Weaver discloses a jig for forming trusses which includes a block designed to fit within the rails of a C-shaped channel that extends along the length of the surface of a truss table. The C-shaped channel, which is recessed within adjacent panels of the table, includes inwardly-directed lips that capture the aforementioned block. A disk-shaped stop is connected with the block by a threaded clamping bolt. The jig can be moved along the length of the C-channel and fixed at a predetermined location in which the stop can define a portion of the truss. A measuring scale fits atop and extends along the length of one of the C-channel rails, and a pointer extends from the block toward the scale to provide the operator with a reference point for positioning the jig during set-up. Another exemplary set-up jig configuration is shown in U.S. Pat. No. 4,943,038 to Harnden. This truss assembly apparatus includes a jig positioned upon a worm gear located within a C-channel. Rotation of the worm gear causes the jig to translate within the C-channel to the desired predetermined location for set-up.
Each of these C-channel-based set-up jig configurations requires that the C-channel be positioned below the table surface so that the top lips of the C-channel are level with the table surface. Although this configuration is suitable for truss tables with wooden table surfaces, it is not particularly suitable for the newer, more preferred steel-topped tables. The table surface of a steel-topped table has a relatively thin depth profile (compared to wooden table surfaces) and is preferably provided to the operator as a single slab; this can preclude the recessing of a C-channel therein.
Another significant shortcoming of these jigging systems is that the top lips of the C-channel of Weaver and the teeth of the worm gear of Harnden are prone to deflect permanently when under stress. Such stress often results when slightly warped planks are bent and forced into place after the set-up jigs are positioned. The forces exerted on the stops of the jigs by the deflected planks can easily be of sufficient magnitude to cause the lips of the C-channel to deform permanently. The deformation can be sufficiently great that the jig contained therein can no longer move freely within the C-channel, and thus is no longer usable. Deformation of the C-channel lips is particularly likely when the set-up jigs are used for both set-up and manufacturing processes. As a result, the C-channel-based jigging systems are only suitable for the set-up process and should not be used as semi-permanent locator stops.
Another set-up jig design suggested for use with steel-topped tables is illustrated in U.S. Pat. No. 5,385,339 to Williams. The Williams set-up jig is a steel block having a recess on its lower surface that mates with a thin metal ruler that extends across the length of the truss table. The steel block slides upon the ruler and is fixed into a predetermined position by a bolt inserted through a laterally positioned aperture in the block and into one of the grid holes of the truss table. The most serious shortcoming of this jigging system is the susceptibility of the ruler to become bent along its edges by contact with truss planks; once this occurs, the steel block no longer slides smoothly on the ruler. Also, this system is intended to be used for set-up alone and not actual truss manufacture.
Another difficulty faced by truss manufacturers is the alignment of adjacent truss tables. Truss tables are typically constructed with relatively few C-channel rails (perhaps 6 to 8 per table) spaced at intervals of about 6 to 10 inches. Of course, a typical truss (e.g., one that is 45 feet in length) cannot fit onto a table of this size. As a result, truss manufacturers employ several truss tables positioned side-by-side to support the entire truss. The employment of multiple tables that include some type of positioning system, such as that of Weaver, Harnden, or Williams, requires that each of the tables be precisely aligned or registered with the remaining tables in order for the positioning system to be useful; otherwise, the positioning system will indicate a position for the locator stops of some tables that is longitudinally offset from proper alignment. Because truss tables can be quite heavy, it is somewhat difficult to position them precisely. Also, it may not be apparent to the operator if the tables become misaligned during repeated operations. The prior art is silent regarding the registration of adjacent tables.
It has been suggested that, instead of shifting the tables to their proper positions, a laser image of the truss be projected onto the table surfaces. See U.S. Pat. No. 5,388,318 to Petta. The use of an overhead imaging system eliminates the need for precise table positioning. However, these systems are quite expensive, and thus are not practical for many truss manufacturers. In addition, the laser image is often difficult to discern unless the truss manufacturing facility is kept quite dark, a working condition that can be difficult or even dangerous. These deficiencies render imaging systems inadequate for many manufacturers.
An additional shortcoming of prior truss tables comes to light when one or more of the truss planks is somewhat bowed or warped. As noted above, after the set-up process is complete, semi-permanent stops are placed around the truss planks to press on the planks and thereby "tighten" the joints of the truss. These semi-permanent stops are positioned, oriented and secured to the truss table based on the shape of the truss planks used for set-up. If some planks used either for set-up or in a subsequent truss are bowed or warped, the placement of the semi-permanent stops may render placement of the later-used planks within the pattern defined by the stops difficult, if not impossible. The prior art is silent regarding apparatus to address the problem of warped planks.
In view of the foregoing, it is a first object of the present invention to provide a truss plank positioning system suitable for use with the more preferred steel-topped truss tables.
It is another object of the present invention to provide a set-up jig and table combination that can withstand the rigors of the set-up and the truss formation operations over time despite the inevitable use of slightly warped truss planks.
It is a further object of the present invention to provide a truss table with a positioning system that can be easily and precisely aligned and registered with adjacent truss tables.
It is also an object of the present invention to provide a truss table with a positioning system that can be used in the darkened conditions found in some truss manufacturing facilities.
It is an additional object of the present invention to provide an apparatus for adapting the locator stops of a truss table to account for warpage and bowing in truss planks.